flockfile(3) — Linux manual page


FLOCKFILE(3)              Linux Programmer's Manual             FLOCKFILE(3)

NAME         top

       flockfile, ftrylockfile, funlockfile - lock FILE for stdio

SYNOPSIS         top

       #include <stdio.h>

       void flockfile(FILE *filehandle);
       int ftrylockfile(FILE *filehandle);
       void funlockfile(FILE *filehandle);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       All functions shown above:
           /* Since glibc 2.24: */ _POSIX_C_SOURCE >= 199309L
               || /* Glibc versions <= 2.23: */ _POSIX_C_SOURCE
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION         top

       The stdio functions are thread-safe.  This is achieved by assigning
       to each FILE object a lockcount and (if the lockcount is nonzero) an
       owning thread.  For each library call, these functions wait until the
       FILE object is no longer locked by a different thread, then lock it,
       do the requested I/O, and unlock the object again.

       (Note: this locking has nothing to do with the file locking done by
       functions like flock(2) and lockf(3).)

       All this is invisible to the C-programmer, but there may be two
       reasons to wish for more detailed control.  On the one hand, maybe a
       series of I/O actions by one thread belongs together, and should not
       be interrupted by the I/O of some other thread.  On the other hand,
       maybe the locking overhead should be avoided for greater efficiency.

       To this end, a thread can explicitly lock the FILE object, then do
       its series of I/O actions, then unlock.  This prevents other threads
       from coming in between.  If the reason for doing this was to achieve
       greater efficiency, one does the I/O with the nonlocking versions of
       the stdio functions: with getc_unlocked(3) and putc_unlocked(3)
       instead of getc(3) and putc(3).

       The flockfile() function waits for *filehandle to be no longer locked
       by a different thread, then makes the current thread owner of
       *filehandle, and increments the lockcount.

       The funlockfile() function decrements the lock count.

       The ftrylockfile() function is a nonblocking version of flockfile().
       It does nothing in case some other thread owns *filehandle, and it
       obtains ownership and increments the lockcount otherwise.

RETURN VALUE         top

       The ftrylockfile() function returns zero for success (the lock was
       obtained), and nonzero for failure.

ERRORS         top


ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface                     Attribute     Value   │
       │flockfile(), ftrylockfile(),  │ Thread safety │ MT-Safe │
       │funlockfile()                 │               │         │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

       These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is

SEE ALSO         top


COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

                                 2020-06-09                     FLOCKFILE(3)

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